Generally, a drive shaft is one of power transmitting mediums for transmitting rotation force of a driving source to a driven member, and may be divided into a solid drive shaft and a hollow drive shaft.
The hollow drive shaft has smaller weight than the solid drive shaft so as to achieve light weight and also has good characteristics from the point of view of torsional strength.
As a conventional art, a hollow drive shaft formed by a high-frequency quenching method which is disclosed in Korean Patent Publication No. 10-2007-0107140 has technical features that contents of Ti, N and B which have effects on quenching quality are regulated and the quenching is performed through a whole thickness in order to obtain a desired strength.
However, since the quenching is performed through the whole thickness in the prior art, the strength may be increased in case of a hollow drive shaft which has a uniform sectional thickness in a whole length direction and a uniform outer circumferential diameter, but there may be a weak portion due to the differences of a sectional thickness and an outer circumferential diameter in case of a drive shaft which has a non-uniform section thickness in a whole length direction and a non-uniformed outer circumferential diameter so that the fatigue load due to a distortion may be focused on the weak portion so as to have a problem of weak strength although the quenching is performed.